1. Field of the Invention
This invention relates to an LCD (liquid crystal display) projector of two-plate type.
2. Description of Prior Art
Since mass production of LCD projectors began in 1989, many technical problems have been solved. For example, the use of optical lens array integrators or glass rod integrators has overcome the problem of uneven illumination on the LCD panel, as well as energy loss due to the mismatch of the illumination distribution of the light source and the shape of the LCD panel. In addition, the use of polarization conversion techniques has overcome the problem of energy loss due to polarized light absorption. Further, a dichroic mirror has been used to solve the problem of color gradation on the image display. High resolution (such as VGA, SVGA, XGA), high density (e.g., 1.3xe2x80x3, 1024xc3x97768) and high image quality (in which the contrast is larger than 200:1) have been achieved by a high temperature poly-TFT LCD.
Nowadays, an LCD can provide a luminance efficiency of about 10 lm/W. For example, EPSON Co.""s ELP-7300 has the highest efficiency among all displays, including CRT, PDP, LED and FEL, etc. The development of LCD devices today is directed toward a high brightness greater than 1200 ANSI lumens, a high resolution greater than 1280xc3x971024, light weight and compact size.
Conventional LCD projectors are normally either three-plate type or one-plate type, both of which use a transparent type LCD panel. However, in order to maintain the size of the LCD panel as the resolution becomes higher, the transparent LCD panel has to be replaced by a reflective type LCD panel to enlarge the numerical aperture.
A In a reflective type LCD panel, the TFT is formed under the reflective metal layer. Because it is not necessary to form a black matrix to prevent current leakage from the TFT when the TFT is illuminated, each pixel on a reflective type LCD panel has a smaller size and a better numerical aperture than on a transparent type LCD panel. The light collective efficiency of an illumination system has its upper limit when a bulb having a specified arc distance is used to illuminate a certain area under a certain divergent angle.
However, the optical construction of a reflective type LCD device is more complex than transparent type LCD device. Moreover, a reflective type LCD device needs to use a polarization beam splitter. In a reflective type LCD device, the light passing through the polarization beam splitter is provided with a certain polarization, becoming thereby, for example, a P-polarized light beam. The P-polarized light beam is modulated and reflected by the LCD panel and then becomes an S-polarized light beam. Thereafter, the S-polarized light beam is reflected by the polarization beam splitter and is then projected to the screen. In an LCD device, the polarization beam splitter must have an excellent beam-splitting effect across the whole visible range, i.e., 400 nm to 700 nm to avoid color distortion. The beam-splitting ratio of S-polarization and P-polarization must meet the requirements of the LCD device even if there is a large incident angle. A larger incident angle of the LCD panel means a higher light collective efficiency for the display device. However, such an LCD panel is very difficult to design and fabricate. Thus, the polarization beam splitter is the critical element restricting the optical characteristics of the reflective type LCD device.
Since a reflective LCD projector needs to use a polarization beam splitter, the projection lens must have a long rear focus so as to place these elements between the PBS and the LCD panel. The rear focus of a reflective type LCD projector is longer than that of a transparent type. This results in a complex design for the LCD projector.
Referring to FIG. 1, a reflective type LCD projector includes: a light source 10, a pre-polarizer 12, a polarization beam splitter 14, a dichroic prism 16, LCD panels 18axcx9c18c and a projection lens 20. The light source 10 emits a non-polarized light. The non-polarized light is polarized to be a linearly polarized light by the pre-polarizer 12. The linearly polarized light is reflected to the dichroic prism 16 by the polarization beam splitter 14. The dichroic prism 16 reflects the red component and the blue component of the polarized light but passes the green component. The green component, blue component and red component are respectively projected to the LED panels 18axcx9c18c. The LCD panels 18axcx9c18c modulate the light with green, blue and red video signals, respectively. The polarization direction of the above green, blue and red light beams is perpendicular to the original polarization direction after reflection by the LCD panels 18axcx9c18c. Therefore, the green, blue and red lights can transmit through the polarization beam splitter 14 and then are projected to the screen 22 by the projection lens 20.
The conventional reflective LCD projector has a long rear focus and is difficult to design since it has a dichroic prism and a polarization beam splitter. Additionally, the projection lens used in the conventional LCD projector is costly. Further, a color deviation is caused since the input light of the projector is S-polarized light and the output light is P-polarized. Furthermore, there is a spectrum shift between the S-polarized light and the P-polarized light caused by the coatings on the dichroic prism and the polarization beam splitter. Moreover, the polarization beam splitter must be broadband since it has to polarize the whole range of visible light. In other words, the polarization beam splitter is complex and costly. Further, the light collection of the LCD projector is difficult since the numeric aperture (N.A.) of the polarization beam splitter is small.
Accordingly, to overcome the complexity of the prior-art systems, the object of the present invention is to provide an LCD (liquid crystal display) projector of two-plate type, the structure of which is simplified to reduce the number of components so as to facilitate production and reduce cost. This LCD projector is suitable for application to projection monitors.
To achieve the above object, this invention uses two LCD panels to modulate the light of three primary colors. To provide a sufficient degree of brightness, the green light is modulated by one of the LCD panels. The red light and the blue light are alternately provided in a time sequential manner. Therefore, the polarization of the green light is distinct from the polarization of the red light and the blue light. This invention utilizes a dichroic mirror and a polarization-rotating device to separate the green light from the red light and the blue light and to provide the green light with a polarization perpendicular to that of the red light and the blue light. Therefore, the red light and the blue light are reflected by a polarization beam splitter while the green light transmits through. The red light and the blue light are separated in a time field sequential manner.